1. Field of the Invention
The present invention relates to injection molding machines and, more particularly, to a hydraulic mechanism for the clamping attachment of interchangeable injection molding dies to the carrier plates of the die closing unit of an injection molding machine.
2. Description of the Prior Art
An important factor in the cost of injection molding is the machine downtime and the skilled labor which are required in conjunction with the changeover from one injection molding die to another. This cost element becomes more important, as production runs become shorter. Shorter production runs, on the other hand, make it possible to reduce product inventory costs.
It has therefore already been suggested to at least partially mechanize the attachment and removal of interchangeable injection molding dies from the die closing unit of an injection molding machine. An important element of this mechanization is the precise and reliable clamping attachment of the two die halves to the stationary and movable die carrier plates of the die closing unit.
One such attachment mechanism is disclosed in "Kunststoffe" Vol. 70 (1980) Issue 3, pp. 128-131 and in the circular "Engel-Information" A-67-TV-9/81 of the Ludwig Engel KG, 4311 Schwertberg, Austria. This prior art mechanism features four axially oriented clamping pins which are bolted to the back side of each die half, so as to be engageable into matching positioning bores of the associated stationary or movable die carrier plate. Each axial positioning bore is intersected by a radial bore inside which is guided a wedge member which is extendable and retractable by means of a hydraulic actuator. Each positioning pin has a transverse bore with an inner drive taper which cooperates with an outer taper of the wedge member, so that the latter, when advanced into the bore, creates a clamping traction on the clamping pin. The wedge taper may be such that the wedge member and the clamping pin create a self-locking action.
The need for clamping pins which extend from the back plates of the die halves represents a disadvantage, inasmuch as the insertion of the injection molding die requires a much greater opening of the die closing unit as would be the case without the clamping pins. Accordingly, the injection molding die has to be held in alignment with the die carrier plates, while the latter are approached for engagement of the clamping pins into the positioning bores.
A similar arrangement is suggested in U.S. Pat. No. 4,116,599, where each die half carries two axially extending clamping pins which reach through the associated die carrier plate into engagement with transversely slidable wedge members which are arranged on the back side of the die carrier plate. This configuration may make it necessary to insert each die half separately, when the overall axial length of the closed injection molding die exceeds the maximum plate opening of the die closing unit.
An additional shortcoming of these prior art attachment mechanisms relates to the fact that an element of positioning inaccuracy is present, due to the need for a certain engagement clearance between the clamping pins and the positioning bores. Within this clearance, the die halves may be displaced out of alignment by the action of the wedge members on the clamping pins.
It is also known to provide injection molding dies with oversize back plates of standardized dimensions, in line with a trend to apply building block principles to the manufacture of injection molding dies. Standardized back plates are listed in "Normalien fur Formwerkzeuge", Katalog K 400 Europa-Reihe of Firma Sustan, Frankfurt, Germany, and EOC-Normalien of EO-Cummernel KG, Ludenscheid, Germany. Oversize back plates are also disclosed in U.S. Pat. No. 2,398,893.
Another prior art suggestion for a mechanized clamping of the die halves to the die carrier plates involves the use of clamping claws which are rotatably supported on the die carrier plates and which engage an oversize back plate of each die half through rotation and a subsequent axial clamping movement. This mechanism requires a comparatively complex actuating drive, and it has the additional disadvantage of making it impossible to use a self-locking action in the clamped position, due to the need for rotating the clamping claw. The absence of a self-locking action constitutes a risk factor, inasmuch as a hydraulic failure of the actuating drive could cause considerable damage to the injection molding die and to the die closing unit.